The Underdark is a network of subterranean tunnels, caverns, seas, and rivers that spans the entire world. Lightless but far from lifeless, the Underdark is home to a dizzying array of creatures, from the civilized but unforgiving drow to carnivorous monsters that haunt the darkness.
Comprising approximately the first 3 miles of depth, the Upperdark is a realm shared by drow, duergar (gray dwarves) and other dwarves, and deep gnomes. Here the races of the World Above (as Underdark dwellers call the surface lands) intermingle with those who dwell in darkness. Encounters here range from drow or dwarf patrols to all manner of wandering monsters and humanoid hunting parties. The largest concentration of plaguecaves are encountered in the Upperdark.
The larger drow and deep gnome cities are located in this layer, 3 to 10 miles below the surface. This area was relatively insulated from the effects of the Spellplague, so many of the drow cities remained unchanged and plaguecaves are rare here. However, the concentration of faerzress increases dramatically in the Middledark.
Below 10 miles lie the most dangerous and alien reaches of the Underdark—places that even drow seldom venture into. There are more dead ends and isolated vaults in the Lowerdark, and the faerzress here is more powerful, covering larger and larger areas with more arcane effects the deeper one descends. The walls between planes can be thin in the Lower dark, with tunnels reaching into the Nine Hells, the Abyss, the Shadowfell, and other domains of chaos and evil.
Settlements and Features
The Underdark is a whole world of its own, with dangers magical and mundane, and a varied ecology that supports millions of sentient beings and ravenous monsters.
This magical radiation, a remnant of a work of dark high magic, is scattered at random throughout the Underdark. Faerzress interferes with spells and the effects of magic items in often unpredictable ways, but has the most detrimental effect on divinations, teleportation, and most other forms of magical travel.Faerzress can take a variety of forms, from clusters of glowing rocks to a barely discernible change in temperature. Natives of the Underdark know to look for colonies of magivores, a form of life somewhere between a plant and a coral that feeds off the magical radiation. Magivore colonies and their airborne seed spores provide a ready source of food for the lower forms of animal life in the Underdark.
Particularly talented wizards are capable of rituals that can alter the form of a pocket of faerzress in specific ways, but those rituals tend to attract the attention of the creatures that feed on magivores—and the creatures that in turn feed on them.
Earth nodes are valuable sources of raw magic, collected from the deepest core of Toril, which flow into a central point indistinguishable from the surrounding territory. They are difficult to find but enormously valuable. Whole Underdark cities, and no small number of jealously guarded citadels, have been built around earth nodes to harness their power.
Earth nodes sometimes generate random effects, such as creating localized earthquakes or granting a stoneskin effect to creatures nearby. In the last several decades, numerous efforts by drow arcanists have resulted in a network of earth node portals that ignore the effects of faerzress.
The Spellplague passed through the Upperdark, affecting the creatures, and terrain there in many of the same ways it altered the World Above. Plaguecaves are areas where active pockets of Spellplague were trapped in enclosed spaces, over time twisting the natural structure of caves and tunnels into bizarre domains. Here streams flow through the air, water falls upward, and earthmotes float about like fish in a bowl.
Some Underdark residents embrace plaguecaves, especially the spellscarred illithids known as the Mindplagued. Most Underdark residents stay away from plaguecaves—drow in particular avoid them at all cost.
Surely the most dangerous of the Underdark’s natural formations, these deep pits, holes, and chasms don’t just appear bottomless. Some empty into other planes of existence, including realms such as the Abyss itself and the Nine Hells, out of which crawl many twisted creatures that inhabit the Underdark.
Seas, Lakes, and Rivers
Water is plentiful in the Underdark and takes as many forms belowground as it does above. Streams of fresh water meander through tunnels, collecting into pools or joining with other streams to form rivers that empty into lakes or seas to rival some of the larger inland seas of the surface. Some caverns are entirely submerged, home to aquatic monsters and humanoids, and waterbound aboleths.
Though most of the Underdark stays at a constant, cool temperature, with air circulating through shafts and tunnels, parts of it can experience something like weather. Cold areas and hot spots, wind tunnels, and strange, billowing mists appear at random intervals.